The present invention relates to a switching control method and apparatus for a wireless telecommunication system and, in particular, but not exclusively for a cellular telecommunications system.
The use of code division multiple access (CDMA) is currently being proposed for the next generation of cellular telecommunication networks. CDMA uses a digital spread spectrum multiple access technique which it is believed will allow the volume of traffic supported by a cellular telecommunications network to be increased. One of the problems of using CDMA techniques is that of power control. With CDMA systems, it is desired that all the signals reaching a given base transceiver station from all the mobile stations in the cell associated with that base transceiver station have the same level. This permits the number of simultaneous calls which can be supported at the same time to be maximised. In particular, if the level of the received signals from the mobile stations are of the same level, the signal to interference ratio of each signal received at the base transceiver station is minimised. If the signal from a given mobile station is received by the base transceiver station with too low a power level, the bit error rate will be too high to provide a reasonable quality of communication. On the other hand, if the level of the signal received from a given mobile station is too high, interference with the other mobile stations sharing the same channel is increased so that some of these other mobile stations may not achieve an acceptable quality of communication with the respective base transceiver station.
In one method which has been proposed, open loop power control and/or closed loop power control is used to determine the level at which a signal is to be transmitted from a mobile station. In open loop power control, the mobile station determines a value for the power of the signal to be transmitted to the base transceiver station based on the level of the signal received at that mobile station from the base transceiver station. Thus, the strength of the signal received by the mobile station from the base transceiver station is used by the mobile station to adjust the power of its own transmission. In other words, the stronger the signal received by the mobile station, the lower the power used by the mobile station to transmit to the base transceiver station.
In closed loop power control, the base transceiver station measures the power level of a signal received from a given mobile station. This measured power level is compared with a desired power level. Based on this comparison, the base transceiver station will send to the mobile station a power adjustment signal indicating to the mobile station the power level at which the mobile station should transmit signals to the base transceiver station.
In methods which use a combination of the closed loop and open loop power values to arrive at the desired power level, the power adjustment signal provided by the base transceiver station and the open loop estimate arrived at by the mobile station itself are both used to obtain the final value for the power level of the signal to be transmitted by the mobile station to the base transceiver station.
If the power control method uses closed-loop power control with short adjustment intervals, the control mechanism can react to fast-fading. Fast-fading is caused by destructive interference between different reflected paths of the same radio signal. It is highly frequency-selective, and therefore the fast-fading process between different radio frequencies is independent. Thus fading might occur at one frequency and not at another, even if the same paths are used. In frequency division duplexed (FDD) systems, where uplink (the link from mobile station to base station) and downlink (the link from base station to mobile station) transmissions are transmitted on different frequencies, closed-loop power control is required to be able to take the difference between link directions into account.
It is also proposed that some cellular telecommunication systems will use hard handover. In particular, it is believed that hard handover may be simpler to implement in practice with certain proposed applications of CDMA. Handover or handoff is when a mobile station stops communication with one base transceiver station and instead communicates with a different base transceiver station. Typically this occurs when a mobile station moves from one cell into another cell. Hard handover or handoff occurs when the mobile station is only in communication with one base station at a time. In other words, the connection with the first base transceiver station is severed before the connection with the second base transceiver station is made. This contrasts with soft handover or handoff where a mobile station can be in communication, at the same time, with several transceiver stations during the transferring process.
In known handoff or handover methods, the mobile station measures the received strength of reference signals transmitted by the base transceiver stations. The mobile station then sends a report back to the base transceiver station with which it is in communication reporting the signal levels of the reference signal received at the mobile station from that base transceiver station as well as the adjacent base transceiver stations from which it was able to receive the reference signals. Based on this report, the base transceiver station in combination with a mobile switching centre, will decide whether or not the mobile station should remain in communication with the current base transceiver station or whether it should be switched to another base transceiver station.
However, this method of achieving handover or handoff has the disadvantage that only the downlink signal (i.e. the signal from the base transceiver station transmitted to the mobile station) is taken into account. The frequency of the downlink signal is generally different from that used in the uplink signal (i.e. the signal transmitted from the mobile station to the base transceiver station). Accordingly, if there is fading in the uplink signal, but not in the downlink signal, handover will not occur, even if handover is in fact appropriate. Instead, the power control method outlined hereinbefore will try to compensate for the fading in the uplink signal by increasing the transmission power of the mobile station. This can cause unnecessary interference with mobile stations in neighbouring cells or indeed other mobile stations contained in the same cell.
It is therefore an aim of embodiments of the present invention to provide a method and apparatus which is able to reduce or at least mitigate the problems described hereinbefore.
According to one aspect of the present invention, there is provided a switching method for a wireless telecommunication system comprising at least one first station and at least one second station, said method comprising the steps of determining at a first station the power level of a signal received from a second station, said first and second stations being in communication; providing a first value for the power level of the signal to be transmitted from the first station to the second station based on said determined power level of the signal received from said second station; determining at the second station the power level of the signal received from said first station; providing a second value for the power level of the signal to be transmitted from the first station to the second station based on said determined power level of the signal received from the first station; comparing said first and second values and based on said comparison determining if said second station is to be switched.
This method is advantageous in that the comparison of the two values takes into account the path between the first station and the second station and the path between the second station and the first station. In other words, if the behaviour of the path in an uplink direction is quite different from that in a downlink direction, it is clear that one of the directions of communication is not performing well. If the performance is unsatisfactory, then switching of the second station can be implemented to try to achieve satisfactory communication in both the uplink and the downlink directions.
Preferably in the comparing step the difference between said first and second values is determined. Alternatively in the comparing step, the difference between the logarithms of the first and second values is determined. It is preferred that said second station be switched if said difference falls outside a predetermined range.
Preferably, the telecommunication system is a cellular telecommunication system. The second station is preferably a base transceiver station or a mobile station. It is most preferably a base transceiver station. The first station may be a mobile station or a base transceiver station. Preferably, the first station is a mobile station. The cellular telecommunication network preferably uses code division multiple access. In a particularly preferred embodiment of the present invention, wide band code division multiple access is used. Wide band code division multiple access allows different band widths to be used in the communication between the base transceiver station and the mobile station in dependence on the bit rate of the data to be transmitted.
Preferably, in a packet data transmission mode, packet data can be transmitted between the first and second stations in a dedicated channel. Additionally or alternatively, in a packet data transmission mode, packet data can be transmitted between said first and second stations on a common channel. It is preferred that the method include the step of selecting either the common channel or the dedicated channel in dependence on the length and/or the frequency of the data packets. It is advantageous to use a dedicated channel when the data packets are long and/or arrive frequently. However, it is advantageous to use the common channel when the data packets are short and arrive infrequently. In this way, the maximum amount of traffic can be supported by the cellular telecommunication system.
Preferably, when said second station switching is activated, the first station ceases to communicate with said second station and starts to communicate with a different second station. In the context of telecommunications, handover or handoff thus occurs. The communication between the first station and the second station may be terminated before communication commences between the first station and said different second station. In the context of cellular telecommunication systems, hard handover or handoff thus occurs. However, it should be appreciated that in some embodiments of the present invention, soft handover or handoff may occur. The use of hard handover is particularly preferred with embodiments of the present invention which use the packet data transmission method in which common and dedicated channels are used. This is because it is difficult to have a random access channel which would be in soft handover. In embodiments of the invention, fast switching between a common channel and a dedicated channel may be used. Soft handover for the dedicated channel would require a more complicated protocol which may be undesirable.
Preferably, when said second station switching is activated, the second station breaks its connection with said first station and makes a new connection with said first station. The new connection may only be successively re-established when an acceptable quality of communication is possible.
Preferably, the method includes the steps of measuring at the first station the strength of reference signals received from a plurality of said second stations, including the second station with which the first station is currently in communication; and terminating the connection between the first station and the second station with which the first station is currently in communication if a stronger signal is received from another of said second stations. Thus, two different criteria can be used for determining whether or not the first station is to communicate with a different second station. In preferred embodiments of the present invention, if either of the criteria is satisfied, then the first station will communicate with a different second station. Preferably second station switching of the type where a connection with the mobile station is broken and re-established will only take place if the result of the comparison of the first and second values is indicative of a failure in the path between the second station and the first station and the current second station is providing the strongest signal, then the connection between the first station will be broken and then subsequently re-established.
Preferably if the comparing step determines that the first and second values are such that the second station should be switched, switching of the second station is delayed until the first and second values have maintained values indicative that switching should take place for a predetermined length of time.
Alternatively, switching can take place immediately that the comparing step determines that the first and second values are such that the second station should be switched.
Preferably said second station is able to determine if the first station is moving and the predetermined time is set in dependence on the length of time required by the second station to determine if the first station is moving. The second station may be able to measure the speed of the first station and the predetermined length of time may be set in accordance with the ability and/or accuracy of the second station to measure the speed of the first station.
Alternatively, if the comparing step determines that the first and second values are such that the second station should be switched, said first station and said second station only provide signals required to determine said first and second values and if the first and second values continue to be such that the second station should be switched, said second station is switched. If the comparing step determines that the first and second values are such that second station switching is no longer required, the connection with the second station may be resumed.
According to a second aspect of the present invention, there is provided a switching method for use in a cellular telecommunication system comprising at least one first station and at least one second station, the method comprising the steps of estimating for the first station a closed loop power level for transmission of a signal from the first station to a second station; estimating for the first station an open loop power level for a signal to be transmitted from the first station to the second station; comparing said estimated open loop and closed loop power levels, wherein if the difference between said estimated values falls outside a given range, second station is switched.
According to a third aspect of the present invention, there is provided a first station for use in a wireless telecommunication system, said first station being arranged to communicate with a second station, said first station comprising means for estimating the power level of a signal received from said second station; means for receiving a value for the power level of a signal to be transmitted from the first station to the second station from said second station; and means for comparing the value determined at said first station with the value transmitted from the second station and based on said comparison, said first station is arranged if appropriate to send a signal to said second station indicative that second station switching is to take place.